1. Field of the Invention
The present invention relates to the field of automatic transmissions for motor vehicles. More particularly it pertains to a method for operating such a transmission having a traction drive variator of the toroidal type.
2. Description of the Prior Art
One type of a continuously variable transmission typically includes a toroidal drive having at least one pair of traction discs, which often react upon each other and are rotatably supported in a housing along an axis facing one another to define a toric cavity between them. A motion transmitting traction roller is disposed in the cavity. The traction roller is frictionally engaged with the discs in circles of varying diameters depending on the transmission ratio, and is so supportive that it can be moved to initiate a change in the transmission ratio. A traction drive continuously variable drive can have more than one cavity and may be used, for example, to form part of an infinitely variable transmission.
A common type of continuously variable transmission includes a toroidal drive having dual cavities, which are defined by two torsionally coupled outboard traction discs, which react upon each other, and two inboard discs, which are positioned between the outboard discs and also react upon each other. One dual cavity toroidal drive of the "off-center type" is disclosed in U.S. Pat. No. 5,368,529. An off-center toroidal drive is usually considered to have an included angle of less than 180 degree between the traction contacts, i.e., where the roller contacts the discs. An on-center toroidal drive is usually considered to have an included angle of about 180 degree. The included angle is the angle formed by the lines between the center of the toric cavity and the traction contact on the engaged discs. The usual method for transmitting power through a dual cavity design of the "off-center type" is to input the power to the two outboard discs and use parallel shafting and gearing to transmit power from the inboard discs.
One gear mesh used to effect this parallel shafting is usually trapped between the inboard discs. Such a two-shaft system is bulky and difficult, if not impossible, to fit into the available space provided for the transmission of a number of vehicles. In addition, it is often necessary to return to the original center line when transmitting power. In the past, this has required a second gear mesh to be used, in addition to the gear mesh between the inboard discs. Single cavity toroidal drives are also known to take up more space than desired.
Therefore, there is a need for a toroidal type transmission capable of inputting and outputting power along the same axis without having to use parallel shafting. U.S. Pat. No. 5,607,372 describes an axial transmission of this type. Such a coaxial-axial drive transmission takes up less space than parallel shaft transmissions and can therefore be used in applications with tighter space constraints. In addition, it is easier and less expensive to package a coaxial-axial drive transmission in a housing than to package a parallel shaft transmission.
Transmissions of this type in the prior art do not have enough speed ratio span to overdrive the variator output. Instead, in infinitely variable transmissions of this type, upon shifting from a low mode to a high mode, the speed ratio produced by the variator is reduced and the transmission speed ratio increases as a result of the increase in the speed ratio of the gearset. To reduce the variator speed ratio the rollers are rotated to the low mode configuration from the higher speed ratio configuration to which they had progressed while accelerating the vehicle from a stop.
Transmissions operating in this way are difficult to control effectively. It is preferred that the variator speed ratio increase continually when the gearset is shifted to a higher speed range without being decreased to a low mode of operation.